Beacon antenna using spiral



A. E. MARsToN BEACON ANTENNA USING SPIRAL July 26, 1960 2 Sheets-Sheet 1 Filed Nov. 28. 1958 INVENTOR T H U R E. MARSTON Ll U S A. KAIS E R,JR.

AR J U ATTORNEY 2 Sheets-Sheet 2 INVENTOR ARTHUR E. MARSTON JULIUS A. KAISER,JR.

ATTORNEY July 26, 1960 A. E. MARSTON ET BEACON ANTENNA USING SPIRAL Filed Nov. 28, 1958 V HH atent A 2.947%!) Patented Jnly26, isso BEACON ANTENNA USING SPIRAL Arthur E. Marston, Alexandria, Va., and Julius A. Kaiser,

, In, Kensington, Md.; said Kaiser assignor to the United States of America as represented by the Secretary of the Navy Filed Nov. 28, 1958, Ser. No. 777,157

6 Claims. (Cl. 343-895) (Granted under Title 35, US. Code (1952), see. 266) 7 The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates in general to antennas and in particular to spiral antennas.

It is an object of the present invention to provide a beacon antenna having atoroidal response pattern in a selected plane.

Another object of the present invention is to provide an antenna having substantially uniform response in all directions in a selected plane.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 shows a spiral antenna assembly which provides a toroidal response with substantially zero response in directions normal to the plane of the spiral.

Fig. 2 shows a view of. the upper or spiral side of spiral antenna element.

Fig. 3 shows a view of the underside of the spiral antenna element showing the feed connector.

Fig. 4 shows typical response patterns for various angles obtained with an antenna constructed in accordance with Figs. 1 through 3.

In many applications of. radio frequency energy operative devices it is desirable to have an antenna which will provide substantially uniform amplitude of response in a selected plane. A typical instance where such operation is desirable is in connection with a beacon system. It is normally characteristic of a beacon system that uniformity of response is' desired for all angles of receipt of energy in one plane, typically the azimuth plane. With such a system therefore, there is no discrimination against signals on a basis of the direction of receipt in the azimuth plane. To achieve a response of this type generally is not particularly dilficult where unlimited space is available because either ,a vertical rod antenna operating against a ground plane or a vertical dipole will provide such omnidirectional response in the selected plane; however, such antennas require considerable vertical height of the lower radio frequencies which in many instances may be undesirable. In contrast,'antennas constructed in accordance with the present invention require a very small amount of vertical height and provide a response which may be termed toroidal in nature having anniform response in one plane, typically an azimuth plane, with substantially zero response in directions perpendicular thereto.

The present invention provides a'spiral antenna element of particular configuration in that it contains two conductors which spiral outward from a central point, the conductors being'relatively insulated from each other the - except at the central pointwhere they are fed in phase typically as by the central conductor of a coaxial feed cable. The spiral antenna element is of an external circumference which is in excess of two wavelengths and less than four wavelengths. Radiation from the central portion of the spiral antenna element is substantially prevented by the placement of a ground plane in close proximity to the spiral conductors in the central region extending to approximately that portion of the spiral wherein the circumference reaches approximately one wavelength. By this means radiation from the central portion is eliminated. Radiation from .the one wavelength circumference portion is eliminated because of the spiral configuration which places the two conductors of the spiral out of phase in the one wavelength circumference region as likewise occurs in the three wavelength circumference region. Radiation thus takes place from the remaining two wavelength circumference region in which the two conductors are substantially in .phase and are not in proximity to a ground plane.

With particular reference now to Fig. 1 of the .draw: ing, shown therein is an overall view of a spiral antenna and its feed system constructed in accordance with the teachings of the present invention. The antenna .consists of a base member 10 of insulating material upon which is disposed the two conductor spiral 11 and 12 wherein the conductors spiral outward from a central portion 13 to the outer periphery. Disc 10 is mounted upon a coaxial cable connector 14 as shown in Fig.3 which is supported by a suitable member 15 and energized through a coaxial cable'16 connected to a radio frequency energy operative device 16-A- On the .underside of disc 10 (Fig. 3) is disposed a disc 17 of conductive material which is separated from the conductors 11 and 12 by the thickness of disc '10 and which forms with the conductors 11 and 12 two transmission lines eliminating radiation from the conductors 11 and 12 in the portions thereofwhich are coextensive with disc 17. Disc 17 is conductively connected to the coaxial cable. connector 14 so ,that it is effectively connected to the outer conductor of the coaxial cable 16. Disc 17 has a radius which is equal to or somewhat less than I providing therefore a circumference which is equal to or somewhat less than one wavelength of the operational frequency.

Conductors 11 and 12 are connected together and fed in phase by the central conductor of the coaxial feed cable 16 at the center point 13. It is noteworthy that an unbalanced to balanced converter is not employed. Since radiation is not desired from the central portion of the spiral, a rapid change outward from the central portion'13 to the 1A circumference is provided as shown in Figs. 1 and 2. It is understood of course that the specific rapid spiral outward in this region is of no particular consequence as long as the two conductors have equal lengths in this region.

As will be presented in more detail in a subsequent point in the specification, as the radio frequency currents progress outward along the spiral conductors, a progressive phase change occurs so that when the region is. reached Where the conductors have an elfective circumference of two wavelengths, they are effectively in phase and hence can provide radiation as an antenna. At portions of the conductors having greater circumference than 2). a progression toward the out of phase condition occurs whichis reached at acircumference of 3).. By further analysis of the foregoing theory it is clear that radiation could also take place at circumferential regions which are even multiples of wavelength, however, in this particular instance where they are not desired it is prevented by restricting the outer circumference of the disc and its conductors 11 and 12 so that the region of 4). is not reached during operation. Such a spiral antenna is inherently a broad frequency band device however since within the limits of above specified it is easily possible, to attain a 2 to 1, frequency band of operation. It is readily apparentjthat the disc structure ofFigs. 1, 2 and 3 can be readily constructed by printed circuit techniques whereinthe disc 10 is of low loss base material and the. conductors, 11 and 12 as well as the conductive disc 17, are conductors remaining after the etch treatment. By such printed circuit techniques it is possible to achieve a high degree of uniformity between spiral antennas produced on a mass production basis.

Although the exact theory of operation of the spiral antenna itself is not universally established at the present time, a possible explanation is that the spiral antenna behaves as a two wire transmission line which gradually by virtue of its spiral geometry transforms itself into a radiating structure or antenna. Ordinarily a two wire transmission line, wherein the Wire spacing is a small fraction of a wavelength, yields a negligible amount of radiation when excited out of phase (by 180?) at its terminals. This is due to the fact that the currents in the two wires of the line in any normal, crosssection are 180 out of phase so that the radiation from one line is essentially cancelled by the radiation from the other. If the two conductors 11 and 12 of the apparatus of Fig. 1 are excited outof phase at the center 13 rather than in phase as shown, and if the spacing betweenadjacent wires is substantially smaller than the radius of the outer turn of the spiral, the difference in length of the two conductors from the origin to a point in the outermost circle is approximately equalto half the circumference of the spiral. With out of phase excitation of the spiral antenna wires at the center, the phasing gradually changes along the length of the two conductors proceeding outwardly so that where the radius of the outer conductor is corresponding to a circumferential distance of one wavelength, the currents in the two conductors are precisely in phase and radiation at a maximum can occur. Such a spiral antenna when excited at higher frequencies wherein the outer conductor radius is greater than would achieve such an in-phase condition at a smaller radius than the periphery so that portions of the conductors located at the smallerradius then produce maxi mum radiation. Such an antenna thus is characterized by wide band operation with respect to frequency because selected portions thereof become effective at different portions of the frequency band. I

Where the basic spiral antenna is fed in phase at the center 13 rather than out of phase as previously described, the central portion itself could normally provide radiation because it is operated substantially in phase. For the purposes of the present invention however, to provide the toroidalresponse pattern, radiationin this central region is eliminated by the conductive ground plane 17; This 'gro'und'plane need extend only to the region of the one wavelength circumference because at that region the currents have progressed to the out-of-pha'se condi tion so that radiation does not occur. Beyond this one wavelength circumferential portion however, application of the foregoinggeneral principles of spiral antennas indicates that the in-phase condition is reached in the circumferential portions wherein the circumference is two wavelengths in magnitude. 7 From directions normal to the pla e of the onductors 11 and 12, all conductors on one side of the central portion 13 have currents which are at all times equalled by oppositely directed currents in diametrically opposed portions of the discs so that the two radiation fields are in opposite phase and the net result is cancellation. Thus it is seen that radiation from directions normal to the plane of the conductors 1'1 and 12 is substantially nonexistent. 1 l

Fig. 4 contains two curves which are plotted to show typical response of an antenna of Figs. 1, 2, and 3 in various angles both in the plane of the conductors 11 and 12 as well as in a plane normal thereto. Curve 20 indicates the typical response to be substantially flat within plus or minus one db in all azimuth directions as one proceeds around disc 10 in the plane thereof. Curve 21 on the other hand indicates a pronounced null in a plane normal to the conductors 11 and 12. From analysis of these two curves 20 and 21 it is readily apparent that a substantially toroidal or doughnut shaped response is achieved, the response being substantially identical both above and below disc 10, as typified for one side by the curve 21.

A It should be understood that the apparatus has been described loosely in terms of response for convenience in explanation of the operation, it being understood that the device is operative for transmission and reception.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A radio frequency system comprising, a radio frequency device, a spiral antenna element having two interspaced conductors with an outer periphery of approximately two wavelengths at thedesired frequency of operation, means for shielding the conductors out to a periphery of approximately one wavelength at the desired frequency of operation, and means for. coupling said element at said device for transfer of radio frequency energy therebetween.

2. An antenna system for a radio frequency device comprising, a spiral antenna element having two interspaced conductors with an outerperiphery of approximatelytwo wavelengths at the desired frequency of operation, means for shielding the conductors out .to a periphery of approximately one wavelength at the desired frequency of operation, and 'means for coupling said element and said means to the radio frequency device for transfer of radio frequency energytherebetween.

3. An antenna system for a radio frequency device comprising, a spiral antenna elementhaving two interspaced conductors withan outer periphery of approximately two wavelengths at the desired frequency of operation, a ground plane disposed in proximity to said element and on the same axis, said ground plane having a periphery of approximately one wavelength at the desired frequency of operation, means connecting the two conductors of the element together at the center of the element, and coupling means connected to the conductors and the ground plane for energy transfer between the' aforementioned components and the radio frequency device.

4..An antenna-system for a radio frequency device comprising, a spiral antenna element having twointerspaced conductors with an outer periphery of approximately two wavelengths at the desired frequency of operation, said conductors having a different pitch in the portion from the vcenter'to the one wavelength periphery than in the portion from one wavelength to two wavelengths, m'eans for shielding the conductors out to a periphery of approximately one wavelength at the desired frequency of operation, and means for coupling said element and said means to the radio'frequencydevice for transfer of radio frequency energy therebetween.

5. Au antenna system for a radio frequency device Comprising a spiral antenna element having two interspaced conductors with an outer periphery of approximately two wavelengths at the desired frequency of operation, a ground plane disposed in proximity to said element and on the same axis, said ground plane having a periphery of approximately one wavelength at the desired frequency of operation, means connecting the two conductors of the element together at the center of the element, and a two conductor coaxial cable having one conductor thereof connected to the conductors of the element and the other conductor thereof connected to the ground plane, said cable being connected to the radio frequency device to provide energy transfer between the aforementioned components and said radio frequency device.

6. An antenna system for a radio frequency device comprising, a circular spiral antenna element having two interspaced conductors with an outer periphery of approximately two wavelengths at the desired frequency of operation, said conductors having a different pitch in the portion from the center to the one wavelength periphery References Cited in the file of this patent UNITED STATES PATENTS 2,611,868 Marston et a1. Sept. 23, 1952 2,850,732 Kandoian et *al. Sept. 2, 1958 2,856,605 Jacobson Oct. 14, 1958 2,863,145 Turner Dec. 2, 1958 OTHER REFERENCES Pub. I, An Experimental Investigation of Cavity- Mounted Helical Antennas, IRE Transactions on Antennas and Propagation, Jan. 1956, pages 53-58. 

